The present invention relates to a starting-point returning apparatus (hereinafter called a "returning apparatus") for a robot, and more particularly, to a returning apparatus for a robot having an improved structure which minimizes the time needed to return the robot's arm to the starting point of a movement cycle.
As shown in FIG. 1, an industrial robot generally has a multi-articulated structure where first and second operation arms 20 and 30 are rotatably connected with each other on a support shaft 10. The first operation arm 20 is driven by a first motor 21 and a first reduction gear 22, and the second operation arm 30 is driven by a second motor 31 and a second reduction gear 32. Here, the reduction gears 22 and 32 are for driving the operation arms 20 and 30, respectively, through a reduction of the rotation speed of each of the motors 21 and 31 by a gear ratio of, say, 1/50.
In the industrial robot having such a structure, at an initial driving time, the operation arms 20 and 30 are controlled such that the operation arms return to the starting point in which the operation arms are aligned with each other. The returning apparatus, as shown in FIGS. 1 and 2, comprises a disc 40 fixed on the support shaft 10 and having first, second and third dogs 42, 43 and 44 arranged at the perimeter thereof at 120.degree. intervals and a sensor 41 fixed to a rotation shaft 23 of the first operation arm 20 and rotating about the perimeter of the disc 40. Here, the sensor 41 rotates together with the first operation arm 20.
The first and second dogs 42 and 43 are for limiting the left and right rotation ranges of the first operation arm 20, respectively, and the third dog 44 is for indicating the starting point of the first operation arm 20. For example, the first operation arm 20 rotates clockwise till the sensor 41 senses the second dog 43, whereupon the clockwise rotation stops. Here, the rotation range of the first operation arm 20 is 240.degree. and that of the second operation arm 30 is 280.degree..
Also, the returning apparatus of the second operation arm 30 with respect to the first operation arm 20 has the same structure as above.
The basic operational sequence for returning to the starting point is that the first operation arm 20 first returns to the starting point with respect to the support shaft 10 and then the second operation arm 30 returns to the starting point with respect to the first operation arm 20.
The operation of the conventional returning apparatus having such a structure will now be described with reference to FIGS. 1 and 2. Here, it is assumed that the sensor 41 is disposed between the second and third dogs 43 and 44, and the initial rotating direction for the starting point returning operation is assumed to be clockwise.
Referring to FIG. 2, upon the initial execution of an operation for returning the robot arm to the starting point, the first operation arm 20 rotates clockwise through an arc "a" and stops when the sensor 41 senses the second dog 43. Then, the first operation arm 20 rotates counterclockwise through an arc "b" and stops when the sensor 41 senses the third dog 44, thus reaching the starting point for the first operation arm 20. Accordingly, the total starting-point returning arc of the first operation arm 20 is equal to a+b.
After the first operation arm 20 returns to the starting point as above, the second operation arm 30 returns to its starting point with respect to the first operation arm 20, using the same method.
Meanwhile, when the operation arms 20 and 30 are disposed between the first and third dogs 42 and 44, the returning angle becomes less than 120.degree.. However, when the operation arms 20 and 30 are positioned between the second and third dogs 43 and 44, the returning angle becomes a+b as described above, thus increasing the returning time and the movement range. That is, the returning process is completed only after the operation arms 20 and 30 are moved to one limit position, either the left or right (dog 42 or 43). Thus, the starting-point returning time is long and there is the potential for interference with other peripheral devices disposed in a rotation path of the operation arms 20 and 30.